Human Movement Science

Published by Elsevier
Print ISSN: 0167-9457
Publications
Physiological tremor during movement is characterized by ∼10 Hz oscillation observed both in the electromyogram activity and in the velocity profile. We propose that this particular rhythm occurs as the direct consequence of a movement response planning system that acts as an intermittent predictive controller operating at discrete intervals of ∼100 ms. The BUMP model of response planning describes such a system. It forms the kernel of Adaptive Model Theory which defines, in computational terms, a basic unit of motor production or BUMP. Each BUMP consists of three processes: (1) analyzing sensory information, (2) planning a desired optimal response, and (3) execution of that response. These processes operate in parallel across successive sequential BUMPs. The response planning process requires a discrete-time interval in which to generate a minimum acceleration trajectory to connect the actual response with the predicted future state of the target and compensate for executional error. We have shown previously that a response planning time of 100 ms accounts for the intermittency observed experimentally in visual tracking studies and for the psychological refractory period observed in double stimulation reaction time studies. We have also shown that simulations of aimed movement, using this same planning interval, reproduce experimentally observed speed-accuracy tradeoffs and movement velocity profiles. Here we show, by means of a simulation study of constant velocity tracking movements, that employing a 100 ms planning interval closely reproduces the measurement discontinuities and power spectra of electromyograms, joint-angles, and angular velocities of physiological tremor reported experimentally. We conclude that intermittent predictive control through sequential operation of BUMPs is a fundamental mechanism of 10 Hz physiological tremor in movement.
 
The role of proprioceptive inputs in the control of goal-directed movements was examined, by means of the tendon vibration technique, in 5 to 11-year old children performing a serial pointing task. Children pointed, with movements of various amplitudes and at various positions, by alternating wrist flexions and extensions. Tendon vibration was applied to both agonist and antagonist muscles to perturb relevant muscular proprioceptive inputs during the static or dynamic phase of the task, i.e., during stops on targets or during movement execution. Constant and variable amplitude errors as well as constant position error were evaluated. Vibratory perturbation applied during movement execution resulted in a similar reduction in movement amplitude, yielding an increased constant error in all age groups and a systematic position error in the direction of the movement starting point. Perturbing proprioception during static phases preceding movement resulted in an age-related increase in the variable amplitude error, which was maximal in 5-year old children performing extension movements. The results were interpreted in terms of the use of proprioceptive information in the feedforward and feedback based components of movement control in children. In particular, the results indicated (1) developmental changes in the relative weighting of each component, (2) an increased capacity to move from one strategy to the other, depending on the availability of information, and (3) developmental changes from an alternated to an integrated control of amplitude and position in serial pointing.
 
Previous paradigms have used reaching movements to study coupling of eye-hand kinematics. In the present study, we investigated eye-hand kinematics as curved trajectories were drawn at normal speeds. Eye and hand movements were tracked as a monkey traced ellipses and circles with the hand in free space while viewing the hand's position on a computer monitor. The results demonstrate that the movement of the hand was smooth and obeyed the 2/3 power law. Eye position, however, was restricted to 2-3 clusters along the hand's trajectory and fixed approximately 80% of the time in one of these clusters. The eye remained stationary as the hand moved away from the fixation for up to 200 ms and saccaded ahead of the hand position to the next fixation along the trajectory. The movement from one fixation cluster to another consistently occurred just after the tangential hand velocity had reached a local minimum, but before the next segment of the hand's trajectory began. The next fixation point was close to an area of high curvature along the hand's trajectory even though the hand had not reached that point along the path. A visuo-motor illusion of hand movement demonstrated that the eye movement was influenced by hand movement and not simply by visual input. During the task, neural activity of pre-motor cortex (area F4) was recorded using extracellular electrodes and used to construct a population vector of the hand's trajectory. The results suggest that the saccade onset is correlated in time with maximum curvature in the population vector trajectory for the hand movement. We hypothesize that eye and arm movements may have common, or shared, information in forming their motor plans.
 
In a sample of 398 twin pairs aged 8-17 belonging to the Italian Twin Registry we explored the extent to which physical clumsiness/motor problems covary with a broad spectrum of behavioral problems identified by the Child Behavior Checklist 6-18/DSM oriented scales, and the causes of such covariation. Only Anxiety and Attention Deficit Hyperactivity (ADH) Problems maintained significant correlation with Clumsiness after partialling out the effects of the other problem scales. By the co-twin control method we found no indication of clear, direct causal effect of Clumsiness upon Anxiety or ADH Problems, or vice versa. Twin bivariate analyses showed that the co-occurrence of motor problems and Anxiety/ADH Problems is best explained by genetic factors shared between Clumsiness and the behavioral problems phenotypes.
 
This study investigates the intra-individual stability of the speed of several motor tasks and the intensity of associated movements in 256 children (131 girls, 125 boys) from the Zurich generational study using the Zurich neuromotor assessment battery (ZNA) over a 12-year period from the age of 6 to 18 years. The stability was assessed by correlograms of standard deviation scores calculated from age- and gender-adjusted normative values and compared with standing height and full scale intelligence quotient (IQ). While motor tasks of hand, finger and foot (HFT) and contralateral associated movements (CAM) exhibited a moderate stability (summary measure as correlation coefficients between two measurements made 4 years apart: .61 and .60), other tasks (dynamic balance, static balance and pegboard) were only weakly stable (.46, .47 and .49). IQ and height were more stable than neuromotor components (.72 and .86). We conclude that the moderately stable HFT and CAM may reflect "motor traits", while the stability of the pegboard and balance tasks is weaker because these skills are more experience related and state-dependent.
 
Thaut and Kenyon [Human Movement Sci. 22 (2003) 321] have shown that, in a task requiring tapping in antiphase with a metronome, the response period adapts rapidly to a small (+/-2%) change in the stimulus period, whereas the relative phase between stimulus and response returns to its pre-change value only very gradually. On the basis of these and earlier findings, Thaut and Kenyon argue that period adaptation is rapid and subconscious, whereas phase adaptation is slow and dependent on awareness of a phase error. This interpretation is at variance with results in the literature suggesting that phase correction is rapid and subconscious, whereas period correction is slow and dependent on awareness of a period mismatch. Although differences in terminology (adaptation versus correction) play a role in this conflict, it primarily reflects different conceptions of sensorimotor synchronization and different interpretations of empirical findings. By excluding from their model a central timekeeper or oscillator with a flexible period, Thaut and Kenyon have omitted an essential component of human timing control that is needed for a proper explanation of their results.
 
Our meta-analysis and conclusions favoring positive bilateral movement training effects on stroke motor recovery (Cauraugh, Lodha, Naik, & Summers, 2010) have been questioned by Pollock, Morris, van Wijck, Coupar, and Langhorne (2011). Their letter to the editor focuses on three subtle metaanalysis techniques. This reply addresses each concern, confirms our robust meta-analysis, and steadfastly upholds our findings.
 
We explored a two-dimensional task space variant of the classical rhythmical Fitts' task in which participants were asked to sequentially cross four targets arranged around the extreme points of the major axes of an ellipse. Fitts' law was found to adequately describe the changes in movement time with the variations in task difficulty (ID), but the 1/3 power-law relating curvature and tangential velocity of the trajectory did not resist the increase in ID. Kinematic analyses showed that the behavioral adaptation to the ID resulted in an increase in the contribution of non-linear terms to the kinematics along the two axes of task space. Moreover, a limit cycle model (combining Rayleigh damping and Duffing stiffness, as in one-dimensional Fitts' task) captured such a behavior. In such a context, Fitts' law and the 1/3 power law appear as surface relations that emerge from parametric changes in a dynamical structure that captures the nature of Fitts' task.
 
The velocity-dependent change in rotational axes observed during the control of unconstrained 3D arm rotations may obey the principle of minimum inertia resistance (MIR). Rotating the arm around the minimum inertia tensor axis (e3) reduces the contribution of muscle torque to net torque by employing interaction torque. The present experiment tested whether the MIR principle still governs rotational movements when subjects were instructed to maintain the humeral long axis (SH-EL) as closely as possible to horizontal. With this view, the variability of 3D trajectories of the minimum inertia axis (e3), shoulder-center of mass axis (SH-CM) and shoulder-elbow axis (SH-EL) was quantified using a VICON V8i motion capture system. The axis for which the 3D variability displacement is minimal is considered as the one constraining the control of arm rotation. Subjects (n=15) rotated their arm in two elbow angular configurations (Elb90° vs. Elb140°), two angular velocity conditions (slow S vs. fast F), and two sensory conditions (kinaesthetic K vs. visuo-kinaesthetic VK). The minimum inertia axis e3 is angled 5.4° away from SH-CM axis, and varied from 27° to 15° away from de SH-EL axis, for Elb90° and Elb140°, respectively. We tested whether the participants would be able to maintain the instructed SH-EL rotation axis or if increasing the frequency of the arm rotations would override the initial rotation instructions and cause the limb to rotate around an axis closely aligned with e3. We expected that VK inputs would minimize the variability of the SH-EL axis and that K should facilitate the detection and rotation around e3 at the faster velocity. Taken together the results showed that the initial instruction, favoring rotation around the SH-EL axis, prevented the velocity-dependent change towards the minimum inertia (e3) and/or the mass axis (SH-CM), i.e., use of the MIR principle. However, the variability of the SH-EL axis was significantly increased in the F condition, confirming that arm rotations around the SH-EL axis produces larger mechanical instabilities in comparison to when the arm is rotated around a mass/inertial axis (Isableu et al., 2009).
 
The purpose of the present study was to search for common patterns and for differences in climbing strategies in a group of recreational climbers. Twelve participants were involved in the study. Each participant climbed a simple indoor route consisting of a 3m horizontal shift followed by a 3m ascent for five times. Climbers could choose their own style, their preferred speed and holds. Their motion was recorded through motion capture based on passive markers. Results suggested that two main climbing strategies were used: the first preferring agility over force and the second preferring force over agility. We also found that our best climbers tried to minimize power during all trials.
 
In this study, we investigated and modeled the performance of target pointing hand movements in a hand free, touchless 3D environment. The targets had different positions, sizes and distances. Performance measurements included total movement time and movement trajectories. The total movement time consisted of a "primary submovement time" and a "secondary submovement time". Results indicated that the total movement time for targets with depth in the upper part of the spherical framework (3.10s) was shorter than for targets without depth (3.79s). The time for targets without depth in the lower part of the spherical framework (2.94s) was shorter than for targets with depth (3.57s). Within a 3D perspective display, the perception of distance and size depends on its depth position. Our results confirmed the adequacy of the 3D information in the display by showing the longest total movement time was observed for the reach of the "forward" target (3.94s). Fitts' model explained the total movement time (for targets without depth r(2)=.72; for targets with depth r(2)=.72). This study showed that participants navigated the 3D space naturally and could move the cursor using both sequential a axis moving strategy and a straight line moving strategy. Real-life applications of the proposed method include interface design for 3D perspective displays and hand movements in 3D environments.
 
Repetitive pointing movements to remembered proprioceptive targets were investigated to determine whether dynamic proprioception could be used to modify the initial sensorimotor conditions associated with an active definition of the target position. Twelve blindfolded subjects used proprioception to reproduce a self-selected target position as accurately as possible. Ten repetitions for each limb were completed using overhead and scapular plane pointing tasks. A 3D optical tracking system determined hand trajectory start and endpoint positions for each repetition. These positions quantified three-dimensional pointing errors relative to the target position and the initial and preceding movement repetitions, as well as changes in movement direction and extent. Target position and cumulative start position errors were significantly greater than the corresponding preceding movement (inter-repetition) errors, and increased as the trial progressed. In contrast, hand trajectory start and endpoint inter-repetition errors decreased significantly with repeated task performance, as did movement extent, although it was consistently underestimated for each repetition. Pointing direction remained constant, except for the angle of elevation for scapular plane pointing, which consistently decreased throughout the trial. The results suggest that the initial conditions prescribed by actively defining a proprioceptive target were subsequently modified by dynamic proprioception, such that movement reproduction capability improved with repeated task performance.
 
The study of rapid strokes is a direct or indirect prerequisite in many fundamental research projects, as well as in the design of many practical applications dealing with handwriting. This paper outlines a family of models, derived from the Kinematic Theory of Human Movements. It explains how the nested models in this family can be used coherently, in the context of a multi-level representation paradigm, to analyze both the trajectory and the velocity of strokes with a progressive amount of detail. In the context of a comprehensive survey of previously published work, this paper highlights many new features of stroke production, when the vectorial version of the theory is fully exploited. In this perspective, the Kinematic Theory is depicted as a potential tool to facilitate communications among researchers working in the multi-disciplinary field of Graphonomics.
 
The purpose of this study was to determine the neuromuscular mechanisms of the involved muscles that contribute to the greater positional variability at the ankle joint in older adults compared with young adults. Eleven young adults (25.6±4.9years) and nine older adults (76.9±5.9years) were asked to accurately match and maintain a horizontal target line with 5° dorsiflexion of their ankle for 20s. The loads were 5 and 15% of the one repetition maximum load (1 RM). The visual gain was kept constant at 1° for all trials. Positional variability was quantified as the standard deviation (SD) of the detrended position signal. The neural activation of the tibialis anterior and soleus muscles was quantified as the normalized EMG amplitude, power spectrum density (PSD; EMG oscillations) and coactivation of the two muscles. As expected, positional variability was greater in older adults (older: 0.11±0.06° vs. young: 0.04±0.02°; p=.003). The only significant neural difference occurred for the PSD of the tibialis anterior muscle, where young adults exhibited significantly greater power than older adults from 30-60Hz. The amplified positional variability of ankle joint in older adults was associated with lower power from 30-60Hz oscillations in the tibialis anterior muscle (r(2)=.3, p=.01). These results provide novel evidence that older adults exhibit greater positional variability with the ankle joint relative to young adults likely due to their inability to activate the tibialis anterior muscle from 30-60Hz.
 
The Animal Fun program was designed to enhance the motor ability of young children by imitating the movements of animals in a fun, inclusive setting. The efficacy of this program was investigated through a randomized controlled trial using a multivariate nested cohort design. Pre-intervention scores were recorded for 511 children aged 4.83years to 6.17years (M=5.42years, SD=3.58months). Six control and six intervention schools were compared 6months later following the intervention, and then again at 18months after the initial testing when the children were in their first school year. Changes in motor performance were examined using the Bruininks-Oseretsky Test of Motor Proficiency short form. Data were analyzed using multi-level-mixed effects linear regression. A significant Condition×Time interaction was found, F(2,1219)=3.35, p=.035, demonstrating that only the intervention group showed an improvement in motor ability. A significant Sex×Time interaction was also found, F(2,1219)=3.84, p=.022, with boys improving over time, but not girls. These findings have important implications for the efficacy of early intervention of motor skills and understanding the differences in motor performance between boys and girls.
 
An attempt was made to extend Fitts' law to a three-dimensional movement (pointing) task to enhance its predictive performance in this domain. An experiment was conducted in which 10 subjects performed three-dimensional pointing movements under the manipulation of target size, distance to target and direction to target. As expected, the duration of these three-dimensional movements was rather variable and affected markedly by direction to target. As a result, the variance in the movement times produced was not satisfactorily explained by the conventional Fitts' model. The conventional model was extended by incorporating a directional parameter into the model. The extended model was shown to better fit the data than the conventional Fitts' model, both in terms of r(2) and the standard error of the residual between the measured movement time and the value predicted by model fit.
 
Transversus abdominis (TA), obliquus internus (OI), and obliquus externus (OE) are involved in multiple functions: breathing, control of trunk orientation, and stabilization of the pelvis and spine. How these functions are coordinated has received limited attention. We studied electromyographic (EMG) activity of right-sided muscles and 3-dimensional moments during treadmill walking at six different speeds (1.4-5.4km/h) in sixteen healthy young women. PCA revealed time series of trunk moments to be consistent across speeds and subjects though somewhat less in the sagittal plane. All three muscles were active during ⩾75% of the stride cycle, indicative of a stabilizing function. Clear phasic modulations were observed, with TA more active during ipsilateral, and OE during contralateral swing, while OI activity was largely symmetrical. Fourier analysis revealed four main frequencies in muscle activity: respiration, stride frequency, step frequency, and a triphasic pattern. With increasing speed, the absolute power of all frequencies remained constant or increased; the relative power of respiration and stride-related activities decreased, while that of step-related activity and the triphasic pattern increased. Effects of speed were gradual, and EMG linear envelopes had considerable common variance (>70%) across speeds within subjects, suggesting that the same functions were performed at all speeds. Maximum cross-correlations between moments and muscle activity were 0.2-0.6, and further analyses in the time domain revealed both simultaneous and consecutive task execution. To deal with conflicting constraints, the activity of the three muscles was clearly coordinated, with co-contraction of antagonists to offset unwanted mechanical side-effects of each individual muscle.
 
The aim of this study was to examine the automatic recruitment of the deep abdominal muscles during a unilateral simulated weight-bearing task by elite Australian Rules football (AFL) players with and without low back pain (LBP). An observational cross-sectional study was conducted using ultrasound imaging to measure the thickness of the internal oblique (IO) and transversus abdominis (TrA) muscles. Thirty-seven elite male AFL players participated. Repeated measures factors included 'force level' (rest, 25% and 45% of body weight), 'leg' (dominant or non-dominant kicking leg) and 'side' (ultrasound side ipsilateral or contralateral to the leg used for the weight-bearing task). The dependent variables were thickness of the IO and TrA muscles. The results of this study showed that thickness of the IO (p<.0001) and TrA (p<.0001) muscles increased in response to 'force level'. During the task, the thickness of the IO muscle on the contralateral side of the trunk relative to the leg being tested, increased more in participants with current LBP (p=.034). This pattern was more distinct on the non-dominant kicking leg. Altered abdominal muscle recruitment in elite athletes with low back pain may be an attempt by the central nervous system (CNS) to compensate for inadequate lumbo-pelvic stability.
 
Muscle synergies are important for spinal stability, but few studies examine temporal responses of spinal muscles to dynamic perturbations. This study examined activation amplitudes and temporal synergies among compartments of the back extensor and among abdominal wall muscles in response to dynamic bidirectional moments of force. We further examined whether responses were different between men and women. 19 women and 18 men performed a controlled transfer task. Surface electromyograms from bilateral sites over 6 back extensor compartments and 6 abdominal wall muscle sites were analyzed using principal component analysis. Key features were extracted from the measured electromyographic waveforms capturing amplitude and temporal variations among muscle sites. Three features explained 97% of the variance. Scores for each feature were computed for each measured waveform and analysis of variance found significant (p<.05) muscle main effects and a sex by muscle interaction. For the back extensors, post hoc analysis revealed that upper and more medial sites were recruited to higher amplitudes, medial sites responded to flexion moments, and the more lateral sites responded to lateral flexion moments. Women had more differences among muscle sites than men for the lateral flexion moment feature. For the abdominal wall muscles the oblique muscles responded with synergies related to fiber orientation, with women having higher amplitudes and more responsiveness to the lateral flexion moment than men. Synergies between the abdominal and back extensor sites as the moment demands change are discussed. These findings illustrate differential activation among erector spinae compartments and abdominal wall muscle sites supporting a highly organized pattern of response to bidirectional external moments with asynchronies more apparent in women.
 
The incidence of (a)symptomatic rotator cuff tears is high, but etiologic mechanisms are unclear and treatment outcomes vary. A practical tool providing objective outcome measures and insight into etiology and potential patient subgroups is desirable. Symptomatic cuff tears coincide with humerus cranialization. Adductor co-activation during active arm abduction has been reported to reduce subacromial narrowing and pain in cuff patients. We present an easy-to-use method to evaluate adductor co-activation. Twenty healthy controls and twenty full-thickness cuff tear patients exerted EMG-recorded isometric arm abduction and adduction tasks. Ab- and adductor EMG's were expressed using the "Activation Ratio (AR)" (-1 ≤ AR ≤ 1), where lower values express more co-activation. Mean control AR's ranged from .7 to .9 with moderate to good test-retest reliability (ICC: .60-.74). Patients showed significantly more adductor co-activation during abduction, with adductor AR's ranging between .3 (teres major) and .5 (latissimus dorsi). In conclusion, the introduced method discriminates symptomatic cuff tear patients from healthy controls, quantifies adductor co-activation in an interpretable measure, and provides the opportunity to study correlations between muscle activation and humerus cranialization in a straightforward manner. It has potential as an objective outcome measure, for distinguishing symptomatic from asymptomatic cuff tears and as a tool for surgical or therapeutic decision-making.
 
This study investigated the relationship between motor-reduced visual perceptual abilities and visual-motor integration abilities of Chinese learning children by employing the Developmental Test of Visual Perception (Hammill, Pearson, & Voress, 1993), in which both abilities are measured in a single test. A total of 72 native Chinese learners of age 5 participated in this study. The findings indicated that the Chinese learners scored much higher in the visual-motor integration tasks than in motor-reduced visual perceptual tasks. The results support the theory of autonomous systems of motor-reduced visual perception and visual-motor integration and query current beliefs about the prior development of the former to the latter for the Chinese learners. To account for the Chinese participants' superior performance in visual-motor integration tasks over motor-reduced visual perceptual tasks, the visual-spatial properties of Chinese characters, general handwriting theories, the motor control theory and the psychogeometric theory of Chinese character-writing are referred to. The significance of the findings is then discussed.
 
Able-bodied individuals spontaneously adopt crouch gait when walking with induced anterior trunk flexion, but the effect of this adaptation on lower-limb kinetics is unknown. Sustained forward trunk displacement during walking can greatly alter body center-of-mass location and necessitate a motor control response to maintain upright balance. Understanding this response may provide insight into the biomechanical demands on the lower-limb joints of spinal pathology that alter trunk alignment (e.g., flatback). The purpose of this study was to determine the effect of sustained trunk flexion on lower-limb kinetics in able-bodied gait, facilitating understanding of the effects of spinal pathologies. Subjects walked with three postures: 0° (normal upright), 25±7°, and 50±7° trunk flexion. With increased trunk flexion, decreased peak ankle plantar flexor moments were observed with increased energy absorption during stance. Sustained knee flexion during mid- and terminal stance decreased knee flexor moments, but energy absorption/generation remained unchanged across postures. Increased trunk flexion placed significant demand on the hip extensors, thus increasing peak hip extensor moments and energy generation. The direct relationship between trunk flexion and energy absorption/generation at the ankle and hip, respectively, suggest increased muscular demand during gait. These findings on able-bodied subjects might shed light on muscular demands associated with individuals having pathology-induced positive sagittal spine balance.
 
Several studies have suggested that children with developmental coordination disorder (DCD) have difficulties in the fine-tuning of manual force. However, parameterization of the generated force per se is hard to test under normal circumstances as movement planning and execution are also involved. In the present study, an isometric force production task was used to test the hypothesis that children with DCD have a decreased ability to scale force to a required force level and to maintain steady low to submaximal forces. We also tested if the developmental trends were different between the children with DCD and typically developing (TD) children. Twenty-four children with DCD and 24 matched TD children, divided over three age groups (7-9-11 years) participated in this study. Analysis of the data showed that DCD and TD children are equally able to adapt their generated force to the required levels, however DCD children produced a less steady force, even more variable than in the youngest TD children. These results suggest that problems in force control in children with DCD are caused by a higher level of inherent noise of the output system. Since younger DCD children are much more affected than older ones it is suggested that these children are able to learn a strategy to cope with their increased stochastic variability, especially at higher force levels.
 
In an earlier study using the visually guided pointing task (VGPT) the authors showed that the timing of imagined movement sequences in children with developmental coordination disorder (DCD) does not conform to the conventional speed-for-accuracy trade-off (or Fitts' law [P.M. Fitts, Journal of Experimental Psychology 47 (1954) 381-391]) that occurs when the distance and accuracy requirements of movements are varied [P. Maruff, P.H. Wilson, M. Trebilcock, J. Currie, Neuropsychologia 37 (1999b) 1317-1324]. The present study sought to replicate this earlier finding and to examine (using a weight manipulation) whether this deficit was also attributable to inaccurate programming of relative force. The chronometry of real and imagined movements was investigated in a group of 20 children with DCD aged between 8 and 12 years and a group of controls matched on age and verbal IQ (VIQ). Movement duration was tested for real and imagined movements using the preferred hand, with the VGPT performed under two load conditions: with and without the addition of a weight attached to a pen. Group means of each subjects' mean movement duration were calculated and plotted against target width for each of the four conditions [Movement type (2) x Load (2)] and a logarithmic curve was fitted to the data points. In the control group, the speed-for-accuracy trade-off for both real and imagined performance conformed to Fitts' law under each load condition. In the DCD group only real movements conformed to Fitts' law. Moreover, the effect of load differed between groups--for real movements, movement duration did not differ between load and no-load conditions for either group, while for imagined movements, movement duration increased under the load condition for the control group only. These results replicate and extend the results of our earlier study. This pattern of performance suggests that children with DCD have an impairment in the ability to generate internal representations of volitional movements which may reflect an impaired ability to process efference copy signals. The ability to programme both relative force and timing appears to underly this difficulty. Results have implications for the use of (guided) motor imagery training in order to facilitate the development of motor skill in children with DCD.
 
A previous experiment investigating visuomotor adaptation in typically developing children and children with Developmental Coordination Disorder (DCD) suggested poor adaptation to an abruptly induced visuomotor perturbation. In the current study, using a similar center-out drawing task, but administering either an abrupt or a gradual perturbation, and twice as many adaptation trials, we show that typically developing children are well able to successfully update an existing internal model in response to a 60 degrees rotation of the visual feedback, independent of the perturbation condition. Children with DCD, however, updated their internal map more effectively during exposure to an abrupt visuomotor perturbation than to a gradual one. This may suggest that the adaptation process in children with DCD responds differently to small vs. large steps of visuomotor discrepancies. Given the known role of the cerebellum in providing an error signal necessary for updating the internal model in response to a gradual visuomotor distortion, the results of our study add to the growing body of evidence implicating compromised cerebellar function in DCD.
 
In the earliest stages of motor-skill learning cognitive, visuo-spatial and dynamic processes play an important role. Which of these should be addressed first when children need to learn a new complex movement sequence? This study compares three learning methods in a within-subject design by having 18 good and 18 poor 8-year-old writers master unfamiliar, letter-like patterns by (1) tracing a trajectory on a screen, (2) tracking a moving target (pursuit), and (3) performing the pattern using written explicit instructions. Following each 10-trial learning phase, the children completed a short test phase. Besides errors and kinematic data, Dynamic Time Warping (DTW) was used to calculate the deviation for each pattern from the ideal shape (DTW-distance). As predicted, the number of errors and DTW-distance were very low during the learning phase of the tracing and pursuit conditions and higher in the explicit condition. Conversely, in the test phase, tracing yielded the highest DTW-distance and the explicit condition the lowest DTW-distance and error percentages. The results were remarkably similar for the good and poor writers. The poor learning results of the tracing condition and the good results of the explicit condition have important implications for the teaching of handwriting and remedial therapy.
 
Many activities require simultaneous performance of multiple tasks. Motor redundancy may provide a key mechanism for multitasking, ensuring minimal inter-task interference. This study investigated the effect of performing two supra-postural tasks on postural stability. The component of joint configuration variance (JCV) reflecting flexible joint combinations (V(UCM)) that stabilize the center of mass (CoM) position and the component of JCV leading to variability (V(ORT)) of the CoM were determined using the Uncontrolled Manifold (UCM) approach. Subjects executed a targeting task alone or in combination with a ball-balancing task. UCM analysis revealed that the joints were coordinated such that their combined variance reflected primarily V(UCM), without a substantial effect on CoM position stability. Evidence for this flexible control strategy increased when the ball-balancing task was added to targeting, or when the index of difficulty of targeting increased, both without leading to substantial increases in V(ORT) or CoM position variance. The increase in joint variance when performing additional tasks without affecting adversely CoM position stability supports the hypothesis that the nervous system takes advantage of available motor redundancy for the successful performance of multiple tasks concurrently. Future work is needed to investigate the limits of this control scheme.
 
This study investigated hemispheric differences in utilizing motor abundance to achieve flexible patterns of joint coordination when reaching to uncertain target locations. Right-handed participants reached with each arm to the same central target when its final location was certain or when there was a 66% probability that its location could change after movement initiation. Use of greater motor abundance was observed when participants reached to the central target under target location uncertainty regardless of the arm used to reach. Joint variance associated with variability of movement direction was larger when reaching with the left, non-dominant arm. This arm also exhibited higher hand path variability compared to the dominant arm. These arm differences were not found when the final (central) target location was known in advance. The results provide preliminary evidence for a greater ability of the dominant (right) arm/left hemisphere to decouple directions in joint space. That is, to increase the use of motor abundance without simultaneously inducing unwanted hand path variability requires that joint variations be restricted to a limited subspace of joint space. Hemispheric differences in motor planning did not appear to account for arm differences related to the use of motor abundance.
 
We aimed to evaluate the relationship between gross motor coordination (MC) and academic achievement (AA) in a sample of Portuguese children aged 9-12years. The study took place during the 2009/2010 school year and involved 596 urban children (281 girls) from the north of Portugal. AA was assessed using the Portuguese Language and Mathematics National Exams. Gross MC was evaluated with the Körperkoordination Test für Kinder. Cardiorespiratory fitness was predicted by a maximal multistage 20-m shuttle-run test of the Fitnessgram Test Battery. Body weight and height were measured following standard procedures. Socio-economic status was based on annual family income. Logistic Regression was used to analyze the association of gross MC with AA. 51.6% of the sample exhibited MC disorders or MC insufficiency and none of the participants showed very good MC. In both genders, children with insufficient MC or MC disorders exhibited a higher probability of having low AA, compared with those with normal or good MC (p<.05 for trend for both) after adjusting for cardiorespiratory fitness, body mass index and socio-economic status.
 
Fifty-eight healthy subjects made rapid elbow extensions to a target over 54 degrees. Angular acceleration was measured and surface electromyograms (EMGs) were recorded from the antagonistic muscles using monopolar rather than bipolar electrode configurations. Marked individual differences were found in the peak value of the first derivative of acceleration (dAcc/dt_Pk). The dAcc/dt_Pk correlated with both quantitative and qualitative properties of the agonist EMGs, but not with those of the antagonist EMG. The agonist EMGs, integrated until the moment of dAcc/dt_Pk, were positively correlated with dAcc/dt_Pk. The interval between EMG onset and EMG peak decreased with increasing dAcc/dt_Pk. The duration of the initial negative phase in the EMGs, which was considered to index the time required to recruit high-threshold MUs, decreased with increasing dAcc/dt_Pk. The results indicate that the ability to rapidly accelerate the lower arm varies across subjects, probably due in part to individual differences in the neural capacity to drive the agonists.
 
Heel impact forces may lead to injury as they travel through the human musculoskeletal system. Previous work on the effect that localized muscle fatigue has on the tibial response (shank axial acceleration) to impact was limited because ankle angle was not controlled. The purpose of this study was to compare the tibial response when the tibialis anterior was fatigued and when not fatigued, while participants controlled dorsiflexion angles at impact using visual feedback. Twenty participants (10 male, 10 female; M+/-SD=21.8+/-2.9 years) were strapped supine to a human pendulum apparatus, and instrumented with a low mass accelerometer (affixed medial to the tibial tuberosity). Participant dorsiflexion angle range was recorded by an electro-goniometer, and divided into four angle ranges so tibial response variables (peak tibial acceleration, time to peak acceleration, acceleration slope) could be compared when fatigued and not fatigued. Peak tibial acceleration and acceleration slopes decreased, and time to peak acceleration increased following fatigue, when comparing values across the same dorsiflexion ranges. Dorsiflexion angle alone did not account for differences in tibial response during localized leg muscle fatigue; supporting prior work and suggesting that the muscle and ankle joint become less stiff when fatigued, thereby increasing the lower extremity attenuation capability to heel impacts.
 
The forces produced by the muscles can deliver energy to a target segment they are not attached to, by transferring this energy throughout the other segments in the chain. This is a synergistic way of functioning, which allows muscles to accelerate or decelerate segments in order to reach the target one. The purpose of this study was to characterize the contribution of each lower extremity joint to the vertical acceleration of the body's center of mass during a hopping exercise. To accomplish this, an induced acceleration analysis was performed using a model with eight segments. The results indicate that the strategies produced during a hopping exercise rely on the synergy between the knee and ankle joints, with most of the vertical acceleration being produced by the knee extensors, while the ankle plantar flexors act as stabilizers of the foot. This synergy between the ankle and the knee is perhaps a mechanism that allows the transfer of power from the knee muscles to the ground, and we believe that in this particular task the net action of the foot and ankle moments is to produce a stable foot with little overall acceleration.
 
There is an increasing interest about upper body accelerations during locomotion and how they are altered by physical impairments. Recent studies have demonstrated that cognitive impairments affect gait stability in the elderly (i.e., their capacity for smoothing upper body accelerations during walking) but little attention has been paid to young adults with intellectual disabilities. The purpose of this study was to examine upright stability in young adults with intellectual disabilities during walking, running, and dual-task running (playing soccer). To this aim a wearable trunk-mounted device that permits on-field assessment was used to quantify trunk acceleration of 18 male teenagers with intellectual disabilities (IDG) and 7 mental-age-matched healthy children (HCG) who participated in the same soccer program. We did not find any significant difference during walking in terms of speed, whereas speed differences were found during running (p=.001). Upper body accelerations were altered in a pathology-specific manner during the dual task: the performance of subjects with autistic disorders was compromised while running and controlling the ball with the feet. Differences in upright locomotor patterns between IDG and HCG emerged during more demanding motor tasks in terms of a loss in the capacity of smoothing accelerations at the trunk level.
 
Accidental falls in older individuals are a major health and research topic. Increased reaction time and impaired postural balance have been determined as reliable predictors for those at risk of falling and are important functions of the central nervous system (CNS). An essential risk factor for falls is medication exposure. Amongst the medications related to accidental falls are the non-steroidal anti-inflammatory drugs (NSAIDs). About 1-10% of all users experience CNS side effects. These side effects, such as dizziness, headaches, drowsiness, mood alteration, and confusion, seem to be more common during treatment with indomethacin. Hence, it is possible that maintenance of (static) postural balance and swift reactions to stimuli are affected by exposure to NSAIDs, indomethacin in particular, consequently putting older individuals at a greater risk for accidental falls. The present study investigated the effect of a high indomethacin dose in healthy middle-aged individuals on two important predictors of falls: postural balance and reaction time. Twenty-two healthy middle-aged individuals (59.5 ± 4.7 years) participated in this double-blind, placebo-controlled, randomized crossover trial. Three measurements were conducted with a week interval each. A measurement consisted of postural balance as a single task and while concurrently performing a secondary cognitive task and reaction time tasks. For the first measurement indomethacin 75 mg (slow-release) or a visually identical placebo was randomly assigned. In total, five capsules were taken orally in the 2.5 days preceding assessment. The second measurement was without intervention, for the final one the first placebo group got indomethacin and vice versa. Repeated measures GLM revealed no significant differences between indomethacin, placebo, and baseline in any of the balance tasks. No differences in postural balance were found between the single and dual task conditions, or on the performance of the dual task itself. Similarly, no differences were found on the manual reaction time tasks. The present study showed that a high indomethacin dose does not negatively affect postural balance and manual reaction time in this healthy middle-aged population. Although the relatively small and young sample limits the direct ability to generalize the results to a population at risk of falling, the results indicate that indomethacin alone is not likely to increase fall risk, as far as this risk is related to above mentioned important functions of the CNS, and not affected by comorbidities.
 
The strategies used by individuals to respond to loading perturbations have implications for both musculoskeletal health and statistical data analysis. The purpose was to explore load accommodation strategies during walking with extremity weights carried in different positions. Twenty subjects walked on an instrumented treadmill while carrying 0, 44.5 and 89N at the wrists and ankles. Peak ground reaction force (GRF) during weight acceptance was extracted for analysis. The change in peak GRF due to the addition of weight was calculated and used to quantify strategies. Results indicated that on average GRF increased (p<.05) more than the increase in weight alone in two of three load carriage positions, and ranged from 0.95 to 1.45N/N. The strategy for weights carried at the wrists with the arms unconstrained (M±SD, 1.06±.42N/N) was significantly (p<.017) less than with the wrists constrained (1.35±.56N/N) or with weights carried at the ankles (1.40±.72N/N). Individuals exhibited a range of strategies from greatly increasing to slightly decreasing GRF with the addition of weight. Ninety-six percent of strategies resulted in GRF increases. Subject strategies may affect tissue loading and their presence decreases the validity of group statistical analyses.
 
Although many studies have examined performance improvements of ballistic movement through practice, it is still unclear how performance advances while maintaining maximum velocity, and how the accompanying triphasic electromyographic (EMG) activity is modified. The present study focused on the changes in triphasic EMG activity, i.e., the first agonist burst (AG1), the second agonist burst (AG2), and the antagonist burst (ANT), that accompanied decreases in movement time and error. Twelve healthy volunteers performed 100 ballistic wrist flexion movements in ten 10-trial sessions under the instruction to "maintain maximum velocity throughout the experiment and to stop the limb at the target as fast and accurately as possible". Kinematic parameters (position and velocity) and triphasic EMG activities from the agonist (flexor carpi radialis) and antagonist (extensor carpi radialis) muscles were recorded. Comparison of the results obtained from the first and the last 10 trials, revealed that movement time, movement error, and variability of amplitudes reduced with practice, and that maximum velocity and time to maximum velocity remained constant. EMG activities showed that AG1 and AG2 durations were reduced, whereas ANT duration did not change. Additionally, ANT and AG2 latencies were reduced. Integrated EMG of AG1 was significantly reduced as well. Analysis of the alpha angle (an index of the rate of recruitment of the motoneurons) showed that there was no change in either AG1 or AG2. Correlation analysis of alpha angles between these two bursts further revealed that the close relationship of AG1 and AG2 was kept constant through practice. These findings led to the conclusion that performance improvement in ballistic movement is mainly due to the temporal modulations of agonist and antagonist muscle activities when maximum velocity is kept constant. Presumably, a specific strategy is consistently applied during practice.
 
Cyclic tasks are performed better than discrete tasks in adults but it is unknown whether this advantage is present in children as well. Three age groups of participants (6, 8, and 10 years old) executed cyclic and discrete aiming movements to two differently sized target using a Fitts task to examine the developmental effects on speed/accuracy trade-off. Children showed the same advantage of cyclic over discrete movements as previously demonstrated for adults but at a slower speed. The slope of the speed accuracy trade-off was similar in the three age groups in the cyclic as compared to the discrete control mode, suggesting that children learn both tasks equally well in this age range. The index of performance (IP) increased with age but not differently for the two control modes. Children showed clear differences between the kinematics of discrete and cyclic movements and these differences were similar to those seen in adults. Cyclic movements were faster, had higher IP, showed fewer changes in velocity and were more ballistic. Thus movement execution was different between the two tasks, consistent with the hypothesis that cyclic tasks make use of neural oscillators. The slower movement speed in young children is consistent with their limited ability to use open loop control.
 
This paper reports the results of a model-based analysis of movements gathered in a 4×4 experimental design of speed/accuracy tradeoffs with variable target distances and width. Our study was performed on a large (120 participants) and varied sample (both genders, wide age range, various health conditions). The delta-lognormal equation was used for data modeling to investigate the interaction between the output of the agonist and the antagonist neuromuscular systems. Empirical observations show that the subjects must correlate more tightly the impulse commands sent to both neuromuscular systems in order to achieve good performances as the difficulty of the task increases whereas the correlation in the timing of the neuromuscular action co-varies with the size of the geometrical properties of the task. These new phenomena are discussed under the paradigm provided by the Kinematic Theory and new research hypotheses are proposed for further investigation of the speed/accuracy tradeoffs.
 
The purpose of this study is to examine the effects of a speed or accuracy strategy on response interference control during choice step execution. Eighteen healthy young participants were instructed to execute forward stepping on the side indicated by a central arrow (←, left vs. →, right) under task instructions that either emphasized speed or accuracy of response in the neutral condition. In the flanker condition, they were additionally required to ignore the 2 flanking arrows on each side (→→→→→, congruent or →→←→→, incongruent). Errors in the direction of the initial weight transfer (APA errors) and the step execution times were measured from the vertical force data. APA error was increased in response to the flanker task and step execution time was shortened with a speed strategy compared to an accuracy strategy. Furthermore, in response to the visual interference of the flanker task, speed instructions in particular increased APA errors more than other instructions. It may be important to manipulate the level of the speed-accuracy trade-off to improve efficiency and safety. Further research is needed to explore the effects of advancing age and disability on choice step reaction in a speed or accuracy strategy.
 
This study was designed to determine differences in the proprioceptively guided movements of children (8-10 years) and adolescents (16-18 years). Participants were blindfolded and asked to actively match passively determined target positions of the elbow joint under three matching conditions. Overall, children were less accurate than adolescents in all matching tasks and utilized different kinematic strategies for making the matching movements. Specifically, children made larger absolute errors and utilized matching movements which, compared to adolescents, were of shorter duration and less irregular in terms of their velocity profiles. An assessment of limb asymmetry was also performed revealing a non-dominant arm matching advantage but only for children and only in the task requiring interhemispheric transfer of a memory-based model of limb position. The proprioceptive differences observed in this study are likely the result of experience-driven refinement in the utilization of somatosensory feedback throughout childhood and into adolescence.
 
Myoelectric (EMG) signals are used in assistive technology for prostheses, computer and domestic control. However, little is known about the capacity of controlling these signals. Specifically, it is unclear whether myocontrol, i.e., the control of myoelectric signals, obeys the same laws as motor control. Neurologically intact adult participants performed pointing tasks with EMG signals captured from the forehead or the hand in two modalities (sustained: stabilize the signal amplitude in the target; impulsion: produce an impulse and return to resting level). In the sustained modality, the time to reach the target (reach time) increased logarithmically with target amplitude, which is compatible with the predictions of Fitts' law. The rate of failure was not significantly affected by target amplitude. In the impulsion modality, the reach time and the rate of failure followed a bow-shaped pattern as a function of target amplitude. Stabilization time in the sustained modality followed a convex (bow-shaped) pattern for the forehead and a concave pattern for the hand. This was the only significant effect of electrode placement in this study. These findings suggest that myocontrol obeys laws that are distinct from those determining motor control, and that the muscular and intra-muscular synergies that produce EMG signals are specific of each pointing modality and target amplitude.
 
To determine whether manual incoordination is caused by attention deficit or not, we used an accuracy drawing task as a primary task in dual-task and resistance-to-distraction studies, and examined if thus measured attention could differentiate inattention (IA) and combined (CO) subtypes of ADHD. The secondary tasks and distractions failed to lower the primary task performance in IA, CO and control groups. We also compared the impairment scores of the accuracy drawing tasks from the Movement Assessment Battery for Children [Henderson, S. E., & Sugden, D. A. (1992). Movement assessment battery for children. London: Psychological Corporation.] between the groups with attention deficit hyperactivity disorder (ADHD) and/or developmental coordination disorder-inaccurate drawing type (DCD-ID). There were no group differences in the impairment score between the control and the ADHD groups, and between ADHD and ADHD plus DCD-ID groups. We concluded that inaccurate drawing is not caused by attention deficit, but that it is a manifestation of a motor deficit as a separate entity from attention deficit.
 
Top-cited authors
Jan Piek
  • Curtin University
Sylvia Ounpuu
  • Connecticut Children's Medical Center
Bouwien Smits-Engelsman
  • University of Cape Town
Karl Newell
  • University of Georgia
Susan Crawford
  • Alberta Health Services